The Beginning or the End

I was very young when I saw this film but I remember the drama of it and the dirt and mud in the scenes where I think they were constructing what I now know to be the Los Alamos site. There was a scene where Tom Drake became exposed to the radiation by catching some equipment and saving many lives which described radiation sickness as "I feel dizzy, etc." I understood that very well. I also fell in love with Robert Walker! I do not remember anything about actual bombing, etc. I think I was too interested in the personal side of the story. This is an historic movie because it was one of the very first about the bomb. I wish it were available anyplace?